1. Field of the Invention
The invention relates to an air mass flowmeter having a microelectromechanical sensor element, a heating element, and at least one temperature sensor element. The temperature sensor element is a thermal element. The thermal element comprises a large number of thermocouples connected in series and through which an electric current flows in succession. A thermocouple comprises a first metal conductor and a second metal conductor connected to one another in each case at an end by a connecting point, the first metal conductor and the second metal conductor are formed from different metals.
2. Detailed Description of the Prior Art
Air mass flowmeters are used, for example, in motor vehicles for determining the air mass drawn in by an internal combustion engine. On the basis of information about a drawn-in air mass, which is as reliable as possible, a combustion process can be optimized by an electronic control system of the internal combustion engine to precisely a quantity of fuel to the air mass supplied to the respective combustion chambers. This results in better energy utilization together with a low level of pollution emissions.
DE 44 07 209 A1 discloses an air mass flowmeter inserted into an intake duct for determining an air mass, wherein a defined proportion of the total flow flows through the air mass sensor. The mass sensor is in the form of an insertion-duct air mass flowmeter. The air mass flowmeter comprises a sensor element, which is arranged in a measuring duct, electronics, which are arranged in a housing for evaluating and/or detecting the measurement values from the sensor element, and also an outlet duct on the other side of the sensor element. For a space-saving arrangement, the specified ducts or air-guiding paths are U-, S- or C-shaped, with the result that an apparatus that is compact overall and is designed as an insertion element is formed.
U.S. Pat. No. 5,393,351 A1 discloses an air mass flowmeter in which the temperature sensor elements, which are in the form of thermal elements, in the edge region of the sensor element are protected by additional thermal elements.
DE 100 57 403 A1 discloses a thermoelectric sensor of the thermopile type. The output signals from the thermoelectric sensor of the thermopile type can be set in such a way that a thermoelectric structure for setting the sensitivity is connected in series with a plurality of thermoelectric structures which form a thermopile.
An air mass flowmeter designed in accordance with the WO 03/089884 A1 is in the form of a hot-film manometer has proven effective in principle.
During the development of modern air mass flowmeters that operate on the basis of sensor elements in the form of microelectromechanical systems (MEMS), it has become apparent that the measurement results of the sensor elements are influenced in a particularly disadvantageous way by contamination. Contamination can be caused by, for example, oil droplets in the air mass flow, and results in signal drift in the sensor element over time. The signal drift may lead to incorrect measurement values for the air mass flow. However, sensor elements in the form of microelectromechanical systems have a large number of advantages that should not be dispensed with.